Energy
Estimate backup hours from usable battery capacity and essential appliance load.
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Home battery backup runtime calculator guide
When the grid drops, the question is simple: how long will the battery keep the essentials running? This calculator takes the usable energy in your battery bank, the average load you plan to carry, and an inverter efficiency, and returns the expected runtime in hours. It suits a home battery such as a Powerwall or LiFePO4 wall unit, a solar hybrid inverter with storage, or the IPS or inverter-plus-battery systems common in Bangladesh, India, Pakistan, and Nigeria during load-shedding. Enter usable capacity, not the nameplate: a 12 V, 150 Ah tubular lead-acid battery holds 1.8 kWh nominally but only about 0.9 kWh should be drawn at a 50 percent depth of discharge, while a LiFePO4 pack of the same rating gives 1.4 to 1.6 kWh at 80 to 90 percent. Use the average wattage of the appliances actually running — the essential-loads calculator on this site gives that number — and remember that runtime is an estimate; motors, temperature, and battery age all change it.
Runtime (h) = usable battery energy (kWh) ÷ (average load (W) ÷ 1,000) ÷ (inverter efficiency ÷ 100). The side row “usable output after efficiency” = battery energy × efficiency ÷ 100.
Worked example with the default inputs: a 10 kWh usable battery, a 500 W average essential load, and 90% inverter efficiency. Convert the load to kilowatts: 500 ÷ 1,000 = 0.5 kW. Divide energy by load: 10 ÷ 0.5 = 20 hours. The calculator then divides by the efficiency fraction, 20 ÷ 0.90 = 22.2 hours, and reports 9 kWh as the usable output after efficiency (10 × 0.90). For a conservative plan, treat inverter losses as a reduction instead — 10 × 0.90 ÷ 0.5 = 18 hours — and use the displayed 22.2 hours as the upper bound.
Battery capacity is in kilowatt-hours of usable energy. Convert an amp-hour rating first: kWh = volts × Ah ÷ 1,000 × depth of discharge, so a 12 V 200 Ah lead-acid bank at 50% DoD is 12 × 200 ÷ 1,000 × 0.5 = 1.2 kWh, and a 48 V 100 Ah LiFePO4 pack at 90% is 4.32 kWh. Load is in watts, averaged over the outage. Efficiency is a percentage; modern pure sine wave inverters run 85 to 92 percent, cheaper square-wave IPS units lower. The result is in hours; multiply by 60 for minutes.
This is a modelled estimate, not a meter reading. Real consumption moves with weather, occupancy, appliance age, standing charges, and tariff structure, and equipment rarely runs at its nameplate rating. Use the figure to compare options against each other, then confirm against your own bill and the manufacturer's specification before committing to a purchase. On this page the figure rests entirely on usable battery capacity, average essential load, inverter efficiency, market / jurisdiction, effective date, data source, update owner and review date, so start there if the home battery backup runtime calculator returns something you did not expect.
Good to know: entering the battery’s nameplate kWh or full Ah rating instead of the usable share is the classic error and roughly doubles the predicted runtime on lead-acid. Peukert’s effect cuts lead-acid capacity further at high discharge rates, so a 1.8 kWh battery pushed at 800 W may deliver noticeably less than the 20-hour rating implies. Inverters also draw 10 to 40 W idle even with nothing plugged in, which matters for small banks over long nights. Finally, most hybrid inverters stop discharging at a reserve setting, often 10 to 20 percent, that you should subtract before entering the usable figure.
Energy results depend on tariffs, equipment behavior, region, weather, and installation assumptions. Verify rates and electrical decisions with the utility, manufacturer, or qualified professional.
Sources
How it works
Estimate backup hours from usable battery capacity and essential appliance load. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Usable battery capacity = 10 kWh · Average essential load = 500 W · Inverter efficiency = 90 % → 22.2 hours (estimated backup runtime). Change an input above and this example updates with your numbers.
Common questions
At a 500 W essential load (fridge, lights, router, fans) a 10 kWh battery gives roughly 18 to 22 hours depending on how inverter losses are counted. Running a whole home at 2 kW average cuts that to about 4.5 to 5 hours.
A 12 V 150 Ah tubular battery holds 1.8 kWh, of which about 0.9 kWh is usable at 50% depth of discharge. A 75 W fan plus 30 W of LED lights is 105 W, so 0.9 ÷ 0.105 ≈ 8.5 hours before inverter losses, or about 7 to 7.5 hours after them.
Divide usable energy by load: 5 kWh at 500 W is about 10 hours; at 1 kW about 5 hours; at 250 W (lights, router, phone charging) around 20 hours. Subtract 8 to 15 percent for inverter losses.
Yes. An inverter converting battery DC to household AC loses 8 to 15 percent as heat, so a 90% efficient unit delivers 9 kWh to appliances from a 10 kWh battery. Cheap square-wave IPS units can be 75 to 85 percent efficient and also shorten fan and motor life.
A 1 ton inverter AC averaging about 900 W would use most of a 5 kWh battery in five to six hours, and a fixed-speed compressor needs an inverter rated for its start-up surge. Most IPS installations exclude AC for this reason; a 10 kWh or larger LiFePO4 system with a 5 kW hybrid inverter can handle one small unit.